PTP is used to synchronize a master clock with slave clocks where extreme precision is required, such as in advanced control or cellular communications systems. Where there are a number of potential master clocks, a best master clock algorithm may select the best clock to use as the master based on predetermined requirements. The selected clock is known as the grand master.
As described in the IEEE 1588 standard referenced above, PTP makes use of timestamped synchronization packets to carry timing information over the network to the receiver, where a physical clock signal is recovered using a phase locked loop driving a digitally controlled oscillator. An exemplary prior art PTP clock recovery system is described in U.S. Pat. No. 7,689,854, published Mar. 30, 2010, the contents of which are herein incorporated by reference. Further details are provided in a White Paper by Hirschmann entitled Precision Clock Synchronization—The Standard IEEE 1588, Rev. 1.2, available from Industrial Networking Solutions, Addison, Tex.
Clock recovery often takes place at the boundary between networks. The requirements for boundary clocks are described by ITU recommendation G.8273.2. Prior art clock recovery devices may not be able to meet the extremely strict requirements of the aforementioned ITU recommendation with regard to the ability to reduce residual phase error when used as boundary clocks. In particular, the use of a conventional phase locked loop (PLL) architecture in network synchronization applications is not possible due to the nature and format of the time transport layer. Any overall solution should aim to satisfy certain limits with respect to:                Initial frequency and phase lock interval        Frequency error after frequency lock declaration        Frequency drift after frequency lock declaration        Phase error after phase lock declaration        MTIE (Maximum Time Interval Error) relative to calculation interval        TDEV (Time Deviation) relative to calculation interval        Holdover performance and enter/exit criteria        
In addition to the meeting those limits the solution should aim to be flexible enough to support the following system requirements:                Warm start of servers with given frequency offset        Tolerance and recovery from frequency/phase jumps        Tolerance and recovery from network downtime scenarios        Transient mitigation for loss of reference        Active/Monitor server frequency and phase lock declarations        Reference switching between active and monitor server references (reference denotes PTP reference in this case)        Mode switching between active and monitor server reference (reference denotes PTP reference in this case, servers may have different modes of operation)        Mode switching between PTP server and electrical frequency/phase sources        